1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to an exterior structure of a lithium secondary battery.
2. Description of Related Art
Secondary (rechargeable) batteries are classified into a nickel-cadmium (Ni—Cd) battery, a nickel-hydrogen (Ni-MH) battery, a lithium (Li) battery, a lithium-ion battery depending on an electrodes (intercalation hosts) used therein.
Particularly, since a lithium secondary battery has a drive voltage of 3.6V, which is about three times that of a Ni—Cd battery or a Ni-MH battery, and a high energy density per a unit weight, the lithium secondary battery is being widely adopted in the art.
In the lithium secondary battery, a lithium-based oxide is used as a positive activation material, and a carbon-based material is used a negative activation material. Further, the lithium secondary battery can be classified into a liquid electrolyte battery and a polymer electrolyte battery depending on an electrolyte used therein. The liquid electrolyte battery is often called as a lithium ion battery, and the polymer electrolyte battery is often called a lithium polymer battery. In addition, the lithium ion battery is fabricated in various shapes such as a roll type, a polygonal type, and a pouch type.
Typically, a lithium secondary battery includes: an electrode assembly having positive electrode plates on which positive activation materials are coated, negative electrode plates on which negative activation materials are coated, and separators interposed between each of the positive electrode plates and each of the negative electrode plates for preventing short circuits and allowing only lithium ions to pass through them; a lithium secondary battery case for containing the electrode assembly; and electrolyte injected into the battery case for enabling the lithium ions to travel.
Also, depending on the electrode used therein, the lithium secondary batteries can be classified into a lithium metallic battery and a lithium ion battery that use liquid electrolyte, and a lithium polymer battery that uses a polymer solid electrolyte.
In rectangular or cylindrical type lithium secondary batteries, aluminum or metallic cans are used as a case for containing the electrode assembly and the electrolyte. In pouch type lithium secondary batteries, a pouch is used as a case for containing them.
Typically, a pouch has a multi-layer structure consisting of a metallic foil layer and a synthetic resin layer covering it. In this case, the weight of the battery can be significantly reduced in comparison with a metallic can. In the multi-layered pouch, aluminum is used as a material for the foil. A polymer film included in the inside layer of the pouch multi-layered film protects a metallic foil from the electrolyte and prevents short circuits among the positive and the negative electrodes, and the electrode terminals.
In order to provide a pouch type lithium secondary battery, first, an electrode assembly formed by stacking positive electrodes, separators, and negative electrodes or stacking and winding them is inserted into an opening of the pouch. Then, edges of the opening of the pouch are heated and melted with front and rear pouch films to seal the opening of the pouch containing the bare cell.
Subsequently, accessories or subsidiary structures such as a protection circuit module or a positive temperature coefficient (PTC) element are attached to the bare cell battery, thereby providing a core pack battery.
When the core pack battery is combined with a cover, a complete lithium secondary battery can be obtained. This type of complete structure may be often called a packed battery. A typical packed battery contains a plurality of bare cells and a common protection circuit in a single cover.
In this case, the cover is provided on an outer surface of the pouch to protect the outer surface which may be made of a relatively weak material. They are bonded by applying a thermally melted adhesive between the pouch and the cover and melting the adhesive.
However, such a conventional pouch has a problem that, because a thermally melted adhesive is applied to the pouch and the adhesive is heated, the electrode assembly contained in the pouch may be deteriorated. In other words, the electrode assembly is deteriorated by a high temperature of heat so that this may produce malfunction of a battery, or the electrode assembly may be exploded to cause an accident.
Additional problems may happen when the bare cell battery is floated in the cover and electrical connections are cut off because the cover should be naturally larger than the bare cell to contain the bare cell.